Testing of wire insulation by corona discharge

ABSTRACT

In the apparatus disclosed, flaws in the insulation of a moving strand of insulated electrical wire are detected by means of a corona discharge. The discharge current is shared between the wire and an adjacent electrode so that the proportioning of the current between the electrode and the wire varies as a function of the quality of the insulation. Accordingly, a flaw in the insulation is indicated when the current picked up by the electrode falls below its normal level.

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States Patent:

3) HIGH VOLIAGIE 0.0, SOURCE 3,134,071 5/1964 Wakefield 3,321,703 5/1967Tystewicz Primary Examiner-Gerard R. Strecker ABSTRACT: In the apparatusdisclosed, flaws in the insulation of a moving strand oti insulatedelectrical wire are detected by means of a corona discharge. Thedischarge current is shared between the wire and an adjacent electrodeso that the proportioning of the current between the electrode and thewire varies as a function of the quality of the insulation. Accordingly,a flaw in the insulation is indicated when the cur rent picked up by theelectrode falls below its normal level.

2 FROM COATING t AND DRYING M COUNTER INVENTOR mMFZDOO 02:56 23 IE oz oo20E E MN womzow 0 0 mwES I9: I

TESTING OF WIRE INSULATION BY CORONA DISCHARGE BACKGROUND OF THEINVENTION brushes or so-called hourglass contacts. However, since thesemechanical devices depend upon physical contact with the wire, theyoften create new flaws in the insulation as well as detecting existingflaws.

Among the several objects of the present invention may be noted theprovision of a novel apparatus for detecting flaws in the insulation ofelectrical wire such as magnet wire; the provision of such apparatuswhich does not require physical contact with the wire being tested; theprovision of such apparatus which does not introduce new flaws into theinsulation of the wire being tested; the provision of such apparatuswhich is highly reliable in the detection of flaws; and the provision ofsuch apparatus which is relatively simple and inexpensive.

Other objects and features will be in part apparent and in part pointedout hereinafter.

SUMMARY OF THE INVENTION Briefly, apparatus according to the presentinvention is adapted for detecting flaws in the insulation of a movingstrand of insulated electrical wire. The apparatus employs an electrodewhich is supported adjacent the moving strand and means, including ahigh-voltage source, for producing a relatively constant current coronadischarge adjacent the electrode and the adjacent portion of the movingstrand. A substantial portion of the discharge current is picked up bythe electrode and the strand together, with the proportioning ofdischarge current between the electrode and the strand being variable asa function of the quality of the insulation on the strand. The coronadischarge current picked up by the electrode is sensed, e.g. by aresistor in series with the electrode discharge path, and a flaw issignalled or registered when the corona discharge current picked up bythe electrode falls below a level corresponding to normal insulationquality.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic illustrationof flaw-detecting apparatus according to this invention;

FIG. 2 is a view substantially on the line 2--2 of FIG. 1 showing anelectrode employed in that apparatus; and

FIG. 3 is a schematic illustration of alternate circuitry which may beemployed in the FIG. 1 apparatus.

Corresponding reference characters indicate corresponding partsthroughout the several views of the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, thereis indicated at 11 a moving strand of magnet wire, e.g. a solid copperwire with an insulating coating comprising one or more layers ofvarnish. A convenient point to perform insulation testing is just priorto coiling after the wire has been coated and dried. Thus, the strand 11may be assumed to be somewhere along a manufacturing line, e.g.following the drying ovens and ahead of the winding area. The strand 11is assumed to be grounded as indicated 19, e.g. through the wire sourcereel and the takeup reeling apparatus.

In accordance with the present invention, an electrode 13 is supported,e.g. on suitable insulators 14, adjacent the moving wire strand 11. Inorder to expose only a limited portion of the strand 11 to the coronadischarge described hereinafter, electrode 13 is preferably somewhatconcave or dished with the strand passing through the concave portion ofthe electrode. As illustrated, this may be accomplished by employing agenerally trough-shaped piece of sheet metal as the electrode 13, thetrough being transversely slotted as indicated at 15 in FIG. 2 so thatthe moving strand can pass transversely through the trough. Suitablerollers may be employed to maintain the alignment of the strand with theslot and prevent contact therebetween. The edges of the slot 15 arepreferably rolled downwardly as indicated at 17 in FIG. 1 so that theelectrode 13 itself does not present any sharp edges to the coronadischarge described hereinafter and thus abrupt field gradients areavoided.

A pointed or needlelike discharge electrode 21 is supported adjacent theelectrode 13 and the exposed portion of the strand 11. A coronadischarge is provided at the point of electrode 21 by means of ahigh-voltage DC source 23, a positive potential with respect to groundbeing applied to the discharge electrode 21 through a resistor R1. As isunderstood in the art, the pointed configuration of electrode 21 createsan abrupt field gradient adjacent the point which facilitates a coronadischarge, i.e. a steady generation of ions, without sparking. As isalso understood by those skilled in the art, the resistor R1 aids inproducing a substantially constant current corona discharge fromelectrode 21.

The corona discharge current is picked up, to a variable extent, by boththe electrode 13 and the strand 11. The discharge current is thus, ineffect, shared between these two possible current sinks. The dischargeelectrode 21 is preferably supported so that it is somewhat closer tothe wire strand than to the closest portions of the electrode 13 so thatthe discharge current would preferentially flow to the strand if it werenot for the insulation on the wire.

Electrode 13 is connected to ground through a resistor R2 which providesa means for sensing the portion of the corona discharge current which ispicked up by the electrode. In other words, resistor R2 provides asignal voltage which is proportional to that current. As will beunderstood by those skilled in the art, measuring the pick up electrodecurrent in this way provides an indirect means of measuring, thedischarge current picked up by the strand 11. This connection alsomaintains the electrode 21 near ground potential. The signal generatedacross resistor R2 is applied to a lowpass filter comprising a capacitorC1 and a resistor R3 for removing relatively highfrequency electricalnoise from the signal provided by electrode 13. The relative values ofresistor R3 and capacitor C1 are, however, selected so that pulsatingsignals generated by the passage of flaws in the insulation on strand11, as described hereinafter, are passed substantially unattenuated. Ifdesired, low-pass filtering can instead be provided by means of acapacitor connected directly across resistor R2, the value of thecapacitor being selected to provide the desired rolloff frequency inrelation to the effective source impedance of the electrode incombination with the resistor R2.

As only a relatively small amount of current will typically be availablefrom the electrode, the filtered electrode signal will typically be atrelatively high impedance. An amplifier 27 is therefore employed forproviding a corresponding signal at relatively low impedance. Theamplified electrode signal is then applied, together with apreselectable DC reference voltage, to a comparator amplifier 31. The DCreference voltage is conveniently obtained by means of a potentiometeras indicated at R4, a fixed DC voltage being applied across thepotentiometer. This latter voltage may, in fact, be the supply voltagefor amplifiers 27 and 31 as will be apparent to those skilled in theart. Comparator amplifier 31 operates to energize, and thereby advance,a counter 33 whenever the amplified electrode signal falls below the 13Creference voltage provided by potentiometer R4.

The operation of this apparatus is substantially as follows. When theinsulation on the strand 11 is of normal quality, that is, withoutflaws, a majority of the substantially constant corona discharge currentis picked up by the electrode 13 and thus a substantially constantvoltage is generated across resistor R2. The voltage across capacitor C1and the amplified electrode signal will therefore also be relativelyconstant.

Potentiometer R4 is then adjusted to provide a DC reference voltagewhich is slightly below the amplified electrode signal. Accordingly, inthe absence of flaws, the counter 33 is not energized.

On the other hand, when a flaw in the insulation on the strand 1 1passes through the electrode 13, a significant increment of the coronadischarge current will be diverted to the strand 11 through the flaw inthe insulation. Accordingly, the voltage developed across resistor R2will drop momentarily. The duration of the drop will depend upon thespeed of the moving strand I1 and the length of the portion thereofwhich is exposed to the corona discharge. In any case, however, thevalues of capacitor C1 and resistor R3 which comprise the low-passfilter are selected to pass such pulse signals. Assuming that theamplifier 27 does not invert the electrode signal, the amplifiedelectrode signal applied to the comparator 31 will thus experience anegative going transient. Assuming the potentiometer R4 has beenappropriately set, this transient or pulse will thus cause theenergization of counter 33. It can thus be seen that the counter willregister or count flaws in the insulation on the moving strand 1 1.

Since the apparatus illustrated registers flaws in the insulation on amoving electric wire strand without physically contacting the wire, itcan be seen that the apparatus does not itself generate tlaws. Further,the measuring signal is generated near ground potential therebyfacilitating its handling and measurement to obtain an indication offlaws. This is to be contrasted with measuring potential changes at thedischarge electrode 21.

While a counter, such as that indicated at 33, is a preferred means forregistering flaws occurring in the insulation on magnet wire as it isbeing manufactured, it should be understood that this apparatus may beemployed with other types of wiremanufacturing processes and in suchenvironments other types of signaling, indicating or registering meansmay be appropriate. For example, the quality of the insulation can beregistered on a meter or oscilloscope which provides a visualrepresentation of the value of the electrode signal. Similarly, a flawmay be registered by providing an appropriate electrical signal to othercontrol circuitry.

As it may be desired to count flaws which are in the nature of discretepits or voids in an insulating film without regard to the quality of theinsulating film generally, it may be desirable in certain instances toAC couple the electrode signal to the registering means so that nominalDC variations, e.g. as might be caused by nominal variations in thethickness of the insulating coating, do not cause a flaw indication. InFIG. 3, the amplified electrode signal from amplifier 27 is applied,through a high pass filter comprising a capacitor C2 and a resistor R6,to a Schmitt trigger circuit 35. The relative values of capacitor C2 andresistor R6 are selected to provide a low-frequency rollofi at a pointwhich will pass the transient or pulse generated as a discrete flawpasses through the sensing electrode 13, while blocking nominal DCvariations. The transient signals are sensed and squared by the Schmitttrigger circuit 35 which in turn drives the counter 33 through asuitable power amplifier 37.

While the corona discharge will typically respond to and detect a flawon any side of the wire strand, it should be understood that severaldetectors according to this invention may be used on a signal strand,the discharge electrodes being positioned on different sides of thewire.

As will be understood, the corona-producing voltage which should beapplied to satisfactorily register flaws will depend upon the character,shape and spacing of the electrodes 13 and 21, as well as upon thenominal character of the insulation on the moving strand 11. Forexample, if it is desired to register flaws constituted by voids in onlyone coat of a multiply coated wire, it may be necessary to arrange theelectrodes and select a source voltage which will produce a field in thevicinity of the wire which will break down the insulation if not alllayers are present. Further, while the use of a DC powered corona sourceis preferred, a source providing unidirectional pulses may be readilyused and an AC source maybe used if rectification and suitable band-passfiltering of the pickup electrode signal are provided.

In view of the foregoing, it may be seen that several objects of thepresent invention are achieved and other advantageous results have beenattained.

As various changes could be made in the above constructions withoutdeparting from the scope of the invention, it should be understood thatall matter contained in the above description or shown in theaccompanying drawings shall be interpreted as illustrative and not in alimiting sense.

What is claimed is:

1. Apparatus for detecting flaws in the insulation of a moving strand ofinsulated electrical wire which is substantially at ground potential,said apparatus comprising:

an electrode;

means for supporting said electrode adjacent said moving strand;

means including a source providing a high voltage relative to ground forproducing a relatively constant current corona discharge adjacent saidelectrode and the adjacent portion of said moving strand, a substantialportion of said discharge current being picked up by said electrode andsaid strand with the proportioning of discharge current between saidelectrode and said strand being variable as a function of the quality ofthe insulation on said strand;

means for sensing relative to ground the corona discharge current pickedup by said electrode; and

means for registering a flaw in the insulation of said strand when thesensed corona discharge current picked up by said electrode falls belowa level corresponding to a normal insulation quality.

2. Apparatus as set forth in claim 1 wherein said electrode is concaveand said strand passes through the concave portion of said electrodethereby to restrict the portion of said strand which is exposed to saidcorona discharge.

3. Apparatus as set forth in claim I wherein said electrode is troughshaped and includes a slot extending transversely of the trough shape,said strand passing through the slot.

4. Apparatus as set forth in claim 3 wherein the edges of said slot arerounded away from said corona discharge producing means.

5. Apparatus as set forth in claim 1 wherein said corona dischargeproducing means includes a pointed discharge electrode, the point ofsaid discharge electrode facing the current pickup electrode.

6. Apparatus as set forth in claim 5 wherein said discharge electrode isconnected to said source through a resistor for maintaining thedischarge current at a relatively constant level.

7. Apparatus as set forth in claim 6 wherein said source provides directcurrent.

8. Apparatus as set forth in claim 1 wherein said means for sensing thedischarge current picked up by said electrode comprises a resistancewhich is in series with the path of the discharge current picked up bysaid electrode and which thereby provides an electrode signal which is afunction of the level of current picked up.

9. Apparatus as set forth in claim 8 wherein said strand is grounded andsaid resistor connects said electrode to ground.

10. Apparatus as set forth in claim 8 including means for fil teringrelatively high-frequency noise components from said electrode signal.

11. Apparatus as set forth in claim 8 including means for comparing saidelectrode signal with a preselectable voltage.

12. Apparatus as set forth in claim 11 wherein said means forregistering a flaw comprises a counter and wherein said comparing meansactuates said counter when the electrode signal falls below saidpreselected voltage.

13. Apparatus as set forth in claim 8 including means for blocking theDC component of said electrode signal.

picked up by said electrode and said strand with the proportioning ofdischarge current between said electrode and said strand being variableas a function of the quality of the insulation on said strand;

sensing relative to ground the discharge current picked up by saidelectrode; and

registering a flaw in the insulation of said strand when the senseddischarge current picked up by said electrode falls below a levelcorresponding to a normal insulation quality.

1. Apparatus for detecting flaws in the insulation of a moving strand ofinsulated electrical wire which is substantially at ground potential,said apparatus comprising: an electrode; means for supporting saidelectrode adjacent said moving strand; means including a sourceproviding a high voltage relative to ground for producing a relativelyconstant current corona discharge adjacent said electrode and theadjacent portion of said moving strand, a substantial portion of saiddischarge current being picked up by said electrode and said strand withthe proportioning of discharge current between said electrode and saidstrand being variable as a function of the quality of the insulation onsaid strand; means for sensing relative to ground the corona dischargecurrent picked up by said electrode; and means for registering a flaw inthe insulation of said strand when the sensed corona discharge currentpicked up by said electrode falls below a level corresponding to anormal insulation quality.
 2. Apparatus as set forth in claim 1 whereinsaid electrode is concave and said strand passes through the concaveportion of said electrode thereby to restrict the portion of said strandwhich is exposed to said corona discharge.
 3. Apparatus as set forth inclaim 1 wherein said electrode is trough shaped and includes a slotextending transversely of the trough shape, said strand passing throughthe slot.
 4. Apparatus as set forth in claim 3 wherein the edges of saidslot are rounded away from said corona discharge producing means. 5.Apparatus as set forth in claim 1 wherein said corona dischargeproducing means includes a pointed discharge electrode, the point ofsaid discharge electrode facing the current pickup electrode. 6.Apparatus as set forth in claim 5 wherein said discharge electrode isconnected to said source through a resistor for maintaining thedischarge current at a relatively constant level.
 7. Apparatus as setforth in claim 6 wherein said source provides direCt current. 8.Apparatus as set forth in claim 1 wherein said means for sensing thedischarge current picked up by said electrode comprises a resistancewhich is in series with the path of the discharge current picked up bysaid electrode and which thereby provides an electrode signal which is afunction of the level of current picked up.
 9. Apparatus as set forth inclaim 8 wherein said strand is grounded and said resistor connects saidelectrode to ground.
 10. Apparatus as set forth in claim 8 includingmeans for filtering relatively high-frequency noise components from saidelectrode signal.
 11. Apparatus as set forth in claim 8 including meansfor comparing said electrode signal with a preselectable voltage. 12.Apparatus as set forth in claim 11 wherein said means for registering aflaw comprises a counter and wherein said comparing means actuates saidcounter when the electrode signal falls below said preselected voltage.13. Apparatus as set forth in claim 8 including means for blocking theDC component of said electrode signal.
 14. Apparatus as set forth inclaim 13 including a counter and means for actuating said counter whenthe AC component of said electrode signal exceeds a predetermined level.15. The method of detecting flaws in the insulation of a moving strandof insulated electrical wire which is substantially at ground potential,said method comprising: supporting an electrode adjacent said movingstrand; producing, at a high voltage relative to ground, an essentiallyconstant current electric discharge adjacent said electrode and theadjacent portion of said moving strand, a substantial portion of saiddischarge current being picked up by said electrode and said strand withthe proportioning of discharge current between said electrode and saidstrand being variable as a function of the quality of the insulation onsaid strand; sensing relative to ground the discharge current picked upby said electrode; and registering a flaw in the insulation of saidstrand when the sensed discharge current picked up by said electrodefalls below a level corresponding to a normal insulation quality.